Method of forming a powder coating on a surface



March 31, 1953, L. R. KOLLER.

METHOD OF FORMING POWDER COATING ON A SURFACE Filed July 7, 1950 Inventor y W Mm m r 0 Mi s w B m MH Patented Mar. 31, 1953 Lewis'l t. Keller, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application July 7, 1950, Serial No. 172,449

15 Claims.

1 This invention relates to powder coatings and to methods of making such coatings. The present application is a continuation-in-part of application Serial No. 95,415, filed May 26, 1949, now abandoned, and assigned to the same assignee as the present application.

Heretofo-re, powder coatings have been produced by various methods. One process comprises forming a suspension of the powder to be coated in a suitable fluid vehicle and allowing the desired quantity of powder to settle under the influence of gravity on the coating supporting base. Such a process produces satisfactory coatings when only a gross, relatively porous coating of non-uniform and non-reproducible thickness, and which has'a haphazard distribution of particles, is desired. This method does not lend itself to the manufacture of smooth Coatings in which the powder particles are uniformly deposited in a layer of predetermined thickness.

Another prior art coating method comprises spraying a suspension of powder particles and binder on the base member to be coated. This method again produces a coating which is not easily controlled as to particle distribution and uniformityand magnitude of thickness. In still another representative prior art process a layer of binding materialis applied to a base and the powder-dusted or sprayed thereon. Inherent in this method are the same disadvantages discussed above whereby non-uniform, unreproducible coatings are obtained.

An object of this invention is to provide a powder coating of uniform and selected thickness.

Another object is to provide a method whereby powder coatings having a precisely controlled and uniform reproducible distribution of particles are produced.

Another object is to provide a method whereby powder coatings having monoparticle thick coating layers may be made.

A still further object of this invention is to provide a method of coating articles with powders wherein successive layers of uniform thickness and distribution of selected particles are obtained.

Other objects will become apparent from a consideration of the following description and the drawing in which Fig. 1 is a perspective view of apparatus for carrying out the present invention; Fig. 2 is a perspective view of a plate having thereon a coating prepared according to the present process; Fig. 3 is a partial crosssectional view of a coating produced by the process of this invention; and Fig. 4 is a perspective view of apparatus for coating irregular articles.

It has been found that superior powder coatings may be made by forming a layer of powder particles on the surface of a suitable liquid, transferring the layer to the surface to be coated,

such as a glass plate or disc, and drying.

More particularly, it has been found that smooth and uniformly distributed reproducible powder coatings of controlled thickness may be quickly and easily produced by floating discrete liquid-repellent powder particles on a liquid medium to form a dispersed layer one particle thick compressing the particle layer to form a compact continuous film or layer one particle thick thereafter transferring the compacted layer to a surface desired to be coated, and drying.

The present invention may be applied to powdered materials in general, including both materials of inorganic, mineral, and organic origin,

which are not deleteriously changed by contact with, or dissolved in, the liquid medium. Thus,

powdered mica, titanium dioxide, silica, many powdered resins, and the like,.may bTe "use'd in the present application with water, and other .liquid media. The invention is particularlyapplicable also to that class of materials known as phosphors which are used in making fluorescent screens such as those employed in television picture tubes, and other apparatus where production of light by means of various exciting media is desired. Examples of such phosphors are zinc silicate, zinc sulfide, cadmium sulfide, zinc-cadmium sulfide, magnesium silicate, cadmium borate, and beryllium silicate. Other examples of phosphors may be found in such works as Introduction to Luminescence of Solids, H. W. Leverenz, Wiley, 1950, and Some Aspects of the Luminescence of Solids, F. A. Kriiger, Elsevier, 1948. Phosphor screens formed by the present process are smooth, of uniform thickness, and closely adherent to the base material. They are characterized by good definition, resolving powder and contrast. l

In the practice of this invention, it is necessary that the powdered materials to be coated be naturally repellent or rendered repellent to the liquid carrying medium on which they are floated to prevent their sinking in the medium. While various liquids may be chosen which will not wet certain specific powders, or conversely, powders chosen which will not be wet by a specific liquid, it is preferable to use water as a readily obtainable carrier medium and to treat the powders, if necessary, :to makerthem watenrepellent. By this process, watermay be used .zasthe carrier medium for all powders desired to be coated. If the powder is one which is not wetted :by water, it is, of course, not necessary to treat it with a water-repellent. Iffh'owever,"the-powder is one which is wetted by water, it may be treated with any of the well known ,water repellents, such as aluminum naphthenate,variousstearates and other wax-like materials. Preferred for the water-repellent treatment in connection with the practice of this invention istheuse of 'organo chlorosilanes, including those described in Patnode Patent 2,306,222, assigned to the same .assignee as the present application. .The specific repellent agent preferred is a mixtureof methyl .chlorosilanes comprising about thirty ,mol per .cent .dimeth-yl .dichlorosilane andabout seventy percent methyl-trichlorosilane. .It has been found that 0.1% to 6% .by weight of the organo silanesvmentioned above added to powders will cause them to become water-repellent and to spread readily on a water surface. For the very best results from 1% to 4% by weight of theorgano chlorosilane may be used. The

treating agent may .bead'ded in any preferred manner to the powder .which is kept in a state of agitation ,during addition or mixed after .the material is added. ,Thus, the water-repellent material maybe added drop-by-drop or in small ,incrementsorsprayed .onto the powder or added intthe formsof a vapor. For small batches, the agent may. be. addedto the powderin a jar, after whichthe jar is-closedandshaken. The powder .treated with silicon halide is spread .out in .a .thin layerfor several minutes in theair to allow the hydrogen halide formed to .escape. .The powder particles-may also be made water-repellent .by treating with silicone oil. Usually the silicone .oil is .used as a water emulsion in concentrations of up to.:about 5%. The amountof emulsion-mixed with the powder ranges from about 0.01 "to :DDA per cent-of the aveight of :the powder which .is .then .oven dried, say at '90 C. 10,150 ;C., :or eaindried. While 'iseveral gspecific methods :of :waterproofing the powder are given herein, other well known "methods, which are intended 'to'be covered hereinuwill occur to those skilled in the art.

The powder, which is eitherin'herently waterrepellent or artifica'lly so by'reason of the above zitrea'tment,'may be placed on the water in any convenient way. The water-repellent powder usually spreads readily over the surface ofthe water, even if deposited thereon in lumps. In some cases, .it may be necessary to scatter the powder somewhat to cause it to spread. It is preferred to apply the powder by shaking through ascreen or cloth, thoughan air blown spray, sprinkling or dusting or other well known methods will serve just as well. For a matter of-a minute or soiafter the powder is deposited :on the water surface, it undergoes "a great deal of activity, during whichtime :any aggregates are broken down and a thin film of powder one particle deep is formed on the water surface. However, this film is so sparse or dispersed that the particles therein are relatively widely separated. In order, therefore, to obtain a continuous coating which is one particle deep or thick, it is necessary to compress the water film or coating, or reduce its area, after which a plate or other base material to be coated may be raised upward through the film from underneath the .surface of the ,water, .the,film..adhering to the "surface .of the 'plate.

For specific examples of the operation of the ,present process, reference is made to the drawing. By one method, as illustrated in Fig. 1, azreceritacle .such ;as shown at I is first filled witlrwater 2 which is as free from foreign matter -as possible. .Breferably, the edges of receptacle 1 are-coated withparamn or similar waxy material, as :shown at 3, so that the water level may be carried slightly above the receptacle edges. -In order-to remove any remaining traces of foreign matter such as dirt or grease from the surface of the water 2, a clean bar similar vto barrier 4 is placed crosswise on the .edges of the receptacle ,at one end and drawn over the entire water surface, its leading edge carrying away any foreignmaterial. A clean water surface is necessary .to insure uniform and constant behavior of the powder on the 'water surface. The powder to'be coated, if not inherently waterrepellent, is treated as disclosed above with a water-repellentagent. It'is then deposited "on the water surface by sifting through a screen, sprinkling, dusting, spraying, or by any other convenientmethod. Just enough powder is deposited to give 'adispersedcoating,.such as that shown at 6. .Up to several minutes. are allowed for any aggregates of powder to break up and disperse over the liquid surface. The dispersed coating fip'so obtained. may .be used as..such.if desired, but usually, as in .thecase .of phosphor coatings for fluorescent screens, for example, it is desirable to have a continuous, uniform layer or coating of particles one particle thick or more. It is, therefore, necessary to compress .or concentrate the dispersed coating into a compact aggregation, such as is shown at 6. This may be readily accomplished by drawing relatively together two barriers, such as bars .4 and fnwhich rest on the edges of receptacle I. By so moving the barriers toward one another, the concentration of powder particles per unit area of water surface is increased. If the particle concentration is so increased 'until incipient cracks or wrinkles beginto form, a continuous coating one particle deep, as at i, is attained.

The thoroughly cleaned article 'or base ma- 'terial, such as-glass slide 8'whose upper surface is to'be coated, isnextraised through-the coating powder film 7 from beneath the water surface, thesurface to be coated carrying away the film contacted by its area. The coating thus ,depositedmay be dried by passing over it a gentle current of warm air. A holder such as that shown at 9 may be used to raise plate'or'slide 8 through film 1. As shown, this holder has a shank portion H! to which are attached arms'so shaped that they holdplate B'WlthOllt contacting its upper surface. .These arms are also so bent that they pass under the barrier, such as 4, and permit the slide .3 to be raised vertically through the water surface and film 1 without displacing the barriers or otherwise breaking the film 1 beforea portionof it has been deposited through the powder layer.

above the receptacle edges.

on the upper surface of slide 8. While a particular design of holder has been described, it is to be understood that the holder may be shaped differently to accommodate discs, squares, and other variously shaped articles to be coated singly or together.

In actual practice, it has been found that as the wet film on a slide or other solid surface is dried, a slight compression of the coating occurs which tends to produce wrinkles. These objectionable wrinkles may be held to a minimum or obviated entirely by allowing the film on the water surface to expand slightly, say about onetenth of its compressed area, after it has been compressed as described above. This is very easily accomplished by moving one or both of the barriers 4 or 5 toward its end of receptacle I the proper amount.

A slide having a coating deposited according to the above process has an appearance such as that presented in Fig. 2, the stippling representing a uniform coating l2 of dry particles one layer deep on slide 8, such as is shown in the magnified sectional view in Fig. 3. It will be noted from Fig. 3 that the particles l3 of powder in the coating are arranged uniformly with a minimum of discontinuity over the surface of the base material 8 in a layer one particle thick. The coatings of the present invention are of definite advantage when compared with coatings made by the above-mentioned prior art methods.

Monoparticle thick powder coatings may be also formed on a base material by passing the surface to be coated substantially vertically During the process the powder layer on the liquid is subjected to a constant lateral pressure directed toward the surface to be coated to permit an even, continuous monoparticle coating to be applied to the base material as it is passed upward through the layer. Most of the water or liquid carried onto the base material along with the powder drains off from under the film as the base is elevated. This removal of water by draining from under the coating as the latter is placed on the base substantially reduces the time of drying over that when the surface to be coated is elevated through the liquid layer horizontally or parallel to the layer. By adjusting the lateral pressure on the powder layer as it is transferred from the liquid to the vertically rising surface so that the layer is in continuous contact with the base, irregular shaped surfaces, including those having both protruding and re-entrant surfaces, may be coated with a monoparticle layer of powdered material. Thus, corrugated surfaces, as concave, convex and other varishaped surfaces may be so coated.

For a specific example of the process of coating a non-planar surface, reference is made to Fig. 4. Receptacle l4, similar to receptacle l except that it is usually deeper to accommodate a vertically disposed base, is filled with water [5 as free as possible from foreign materials. Waxy material l6, such as paraflin and the like, is placed on the upper edges of the receptacle to allow the water level to be carried slightly A barrier in the form of a clean metal bar may be drawn over the water surface to further remove any foreign "ready water-repellent, is deposited on the water surface, as described above, to provide a dispersed coating as at H. The dispersed coating H is then compressed to a continuous monoparticle thick layer by drawing together two barriers l8 and I9 which are similar to 4 and 5. Again the presence of a continuous coating one particle thick 20 is denoted by the formation of incipient cracks or wrinkles in the layer surface. The thoroughly cleaned base material, such as the corrugated article 2|, shown extending partly out of the liquid, whose front surface is to be uniformly coated, is next raised vertically upward through the powder layer surface by means of a holder such as that shown at 2!. As the base 2| is slowly raised upward through layer 29, barrier I8 is moved toward barrier 9 to maintain a constant lateral pres,- sure on the film 20 as it is pressed against 2| and to maintain the monoparticle thick nature of the layer on liquid I5 as it is transferred to coat 2!. By maintaining a constant lateral pressure on layer 21! and, hence, an even pressure of layer 20 against the article 2| to be coated, a uniform monoparticle thick layer is deposited on 2| regardless of the shape of the latter. While 2| is shown in the figure as having a corrugated shape, it may be planar or of any irregular shape whatsoever.

As the article being coated rises above the surface of the water, the water or other liquid. carried along with the coating material ontlr the article substantially completely drains off so that very little drying is necessary. Articles so coated take less time to dry than those made as in Fig. 1 in which case the moisture has to evaporate through the coating thickness. The final coating is similar to that shown in Fig. 3 except that the base need not be planar.

Any number of successive coatings may be formed by the method illustrated in Fig. 4 so long as each coating is dried before the succeeding one is applied.

Powder coatings made according to the present process adhere to the material coated without the aid of adhesives and will resist usual handling or jarring so long as the coating itself is not brushed or scraped. If it is desired to add additional layers of the same or a different powder to the base layer, it is best to thoroughly air dry or heat dry the screen or slide with the first layer to waterproof the coating. Once the previously coated layer has been thus treated,

it may be immersed in water without danger of stripping. Any number of layers may be added successively so long as each previous layer is dried as above.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of forming a powder coating one particle thick on a surface which comprises rendering the powder particles liquid-repellent, floating said powder particles on a clean liquid surface to form a dispersed powder layer thereon, reducing the area of the powder layer until the appearance of incipient wrinkles in the layer, passing the surface desired to be coated upward through the powder layer and thereafter drying the coating.

2. The method of forming a powder coating on a surface which comprises rendering powder particles water-repellent, floating the powder particles on water to form a powder layer, compressing the area of the powder layer until the appearance of incipient wrinkles in the layer, releasing the compression to allow about a onetenth. increase in the .compressed area, passing :the surface desir.ed:to be coated .upward through -the ;.pow.der layer and drying the coated surface.

The method .of formingsuccessive coatings one particleithickon a surface which comprises :flo'atingliquids-repellent powder particles .on a .clcanliquid'surface to form a dispersed powder layer thereon, reducing the area of the powder layer until the appearance of incipient wrinkles in .thelayer, enlarging the reduced area about onestenth, passing "the surface to be coated upwardthrough the powder. layer, drying the coatingandrepeating the .coating process.

:4. Thetmethod of forming afluorescent screen coating comprising successive layers of phosphor material-which comprisesfloating liquid-repellent phosphor powder particles on a liquid carrier medium to forma powderlayer, compressing the =area1ofithe powder layer until the appearance .of incipient wrinkles or cracks in the .layer, enlarging the compressed area about one-tenth, passing the surface to be coated upward through the powder layer, drying the coating and .repeating'the process :as desired.

:5. :The method of forming a powder coatingon :an irregularly shaped base member which com- ;prises floating liquid-repellent powder particles on a liquid carrier medium to form .a powder layer, compressing the powder layer laterally to from a compact aggregation of powder particles and thereafter transferring the compacted layer .tO the base member bypassing the surface substantially vertically upward through the powder layer while maintaining a constant lateral pressure on the powder layer directed toward the surface to be coated.

6. The method of forming a powder coating on an irregularly shaped base member which comprises treating discrete powder particles with a liquid-repellent agent, floating the treated particles on aliquid carrier medium to form a powder layer, exerting lateral compression .on the float- .ing-powderzlayer to form it into a-compact aggregation and thereafter transferring the compacted layer to-the surface desired to be coated by passing the surface substantially vertically upward through the powder layer while maintaining a constant lateral pressure on the powder layer directed toward the surface to be coated.

'7. The method of forming a fluorescent screen one particle thick on an irregularly shaped base member which comprises floating liquid-repellent powder particles on a'liquid carrier medium to forma powderlayer, compressing thearea of .thepowderlayer until the appearance of incipient wrinkles therein, allowing the :corrpressed powder layer to expand about one-tenth of its compressed area, and thereafter transferring the powder layer to the base member by passing the surface substantially vertically upward through the powder layer while maintaining a constant lateral pressure on the powder layer directed toward the surface to be coated.

-8. The method of forming a powder coating on a surface which comprises rendering powder particles water-repellent by treating the particles with a material selected from the class consisting of silicone oil and organo chlorosilane, floating the powder particleson water to forma powder layer thereon, compressing the area of the powder-layer until the appearance of .incipent wrinkles in the layer, passing the surface to be coated upward through the powderlayer .anddrying the .coatedsurface.

.9. Themethod .of..forming,a,powder coating .on

a surface which comprises renderingpowderparticles water-repellent .by treating .thepowderparticles with .an .organo chlorosilane, floating the powder particles on water toform a powderlayer thereon, compressing the area of the powder layer until the appearance of incipient wrinkles in the layer, releasing the compression to allow about a one-tenth increase in the compressed area, passing the surface desired to .be coated upward through the powderlayer and drying the coated surface.

.10. The method of forming a powder coating on .a surface which comprises rendering powder particles water-repellent by treating the powder particles with a methyl chlorosilane, floating the powder particles .on water .to.form apowder layer therein, compressing thearea of the powder'layer until the appearance of incipient wrinkles in the layer, releasing the compression to allowabout a one-tenth increase in the compressed area,,passing the surface desired to be coated upward through the powder layer and drying the coated surface.

11. .The method .of forming a powder coating one. surface which comprises rendering powder particles water-repellent .by treating the powder particles with .a.mixture of methyl chlorosilanes comprising about 30. mol per cent dimethyl chlo- .rosilaneand about vl0 percent methyl trichlorolayer until theappearance of incipient wrinkles .111 thelayer, and thereafter transferring the compacted layer to the base member by passing the surface substantially vertically upward through the powder layer while maintaining a constant lateral pressure on the powder layer directed toward the surfaceto becoated.

13. The method of forming a powder coating one surface which comprises rendering the powder particles water-repellent by treating the powder particles with a .mixture of methyl chlorosilanes comprising about 30 mol percent dimethyl dichlorosilaneand about 70 mol per cent methyl .trichlorosilane in the amount of 0.1 per cent to 6 percent by weight based .on the weight of the powder,particles,.floating.the powder particles on water to form .a powder layer thereon, compressing the area of the powder layer until the appearance of incipient wrinkles in the layer, releasing the compression to allow about a onetenth increase .in thecompressed area, passing the surface desired to be coated upward through the powder layer and drying the coated surface.

.14. The method of .forming a powder coating on a surface which comprises rendering powder particles water-repellent by treating the particles with a silicone oil, floating the particles on water to form a powder layer thereon, compressing the area of the powder'layeruntil the appearance of incipient wrinkles therein, passing the surface to be coatedupward through the powder layer-and drying the coated surface.

:15. The method of forming successive-powder coatings one particle thick on a surface which comprises rendering the particles water-repellent by treating with a material selected from the group consisting of silicone oil and organo chlorosilane, floating the particles on water to form a powder layer thereon, compressing the area of the powder layer until the appearance of incipient wrinkles therein, passing the surface to be coated upward through the powder layer and drying the coated surface.

LEWIS R. KOLLER.

REFERENCES CITED The following references are of record in the file of this patent:

Number UNITED STATES PATENTS Name Date Dewar Sept. 9, 1884 Blodgett Nov. 5, 1940 Scott et al Apr. '7, 1942 Dreyer Sept. 25, 1945 Saflord July 29, 1947 Blodgett et al Jan. 10, 1950 

1. THE METHD OF FORMING A POWDER COATING ONE PARTICLE THICK ON A SURFACE WHICH COMPRISES RENDERING THE POWDER PARTICLES LIQUID-REPELLENT, FLOATING SAID POWDER PARTICLES ON A CLEAN LIQUID SURFACE TO FORM A DISPERSED POWDER LAYER THEREON, REDUCING THE AREA OF THE POWDER LAYER UNTIL THE APPEARANCE OF INCIPIENT WRINKLES IN THE LAYER, PASSING THE SURFACE DESIRED TO BE COATED UPWARD THROUGH THE POWDER LAYER AND THEREAFTER DRYING THE COATING. 